There's a long time to go before the SNP's promised referendum on independence, but Alex Salmond aims to use it to exploit the momentum the party has gained from its overwhelming success in the May elections. His latest London-based target is the UK supreme court, which two weeks ago ruled that a Scottish conviction should be overturned on human rights grounds. That, the Nationalists say, fails to honour their nation's established right to conduct its own legal system.

Scotland's determination to march to its own music is the most spectacular sign of the diminishing political unity of the United Kingdom. But it's not alone. Fifty years ago, the Conservatives could be sure of nearly 50% of the vote in Scotland; in the 2010 election, they took 17%, and only one seat. But 50 years ago, too, the party held four of Manchester's nine parliamentary seats, and six of Liverpool's nine. They have not now won in Manchester since 1983, or in Liverpool since 1979. In the local elections in May, they failed to win even one seat in Liverpool, Manchester, Newcastle and Sheffield. Had they not through the Thatcher years forfeited the allegiance of great cities they might not be needing coalition partners today.

Yet these elections were ominous for Labour too. A month ago, Ed Miliband rode in triumph to Gravesham, Kent, to celebrate its capture from the Conservatives. "North, south, east and west," he enthused, "Labour is making gains and coming back." Not so. Of the 27 councils Labour gained, only one apart from Gravesham was in the south. Of 61 councils where Labour failed to take even one seat, almost eight in 10 were southern. As the geographer Danny Dorling demonstrates in his book So You Think You Know About Britain?, in 2010, political polarisation, north versus south, urban versus rural, reached its highest point since 1918. Nor is it hard to see why. As Dorling also shows, British society, as measured by a wealth of indicators from life expectancy to housing provision, is built on inequality. The least favoured places, by no means all in the north, have done worst in recession; but they also gained least from Labour's years of prosperity.

There's a hard choice here for Mr Miliband. He needs to recapture the support which Tony Blair once mustered across the deep blue south. But to do so implies a top priority for what New Labour used to talk of as "southern comfort" – at great possible cost to harder pressed places across the rest of the land. For David Cameron to win back Liverpool Walton, for Ed Miliband to recapture Wimbledon – such epic feats look well beyond their present capacities. We may have rejected AV, but the chance of returning to clear cut national outcomes looks remote.